The letters say that Justice will subpoena the documents if a jurisdiction declines to share. The letter outlines other consequences, too:

Should the Department determine your jurisdiction is out of compliance with section 1373, the Department may, as detailed in your award documents, seek return of your FY 2016 grant funds, require additional conditions for receipt of any FY 2017 Byrne JAG funding for which you have applied, and/or deem you ineligible for FY 2017 Byrne JAG funds.

Justice's moves to clamp down on sanctuary jurisdictions have drawn lawsuits by many of those jurisdictions. They argue, among other things, that Section 1373 amounts to unconstitutional commandeering of local officers, that Justice's conditions on their grants fail the conditioned-spending test under South Dakota v. Dole, and that Justice has no authority to impose conditions on federal grants without Congress's say so. We last posted on the suits here.